A printing apparatus serving as a printer, copying machine, facsimile apparatus or the like or a printing apparatus used as an output device for a composite electronic device or workstation such as a computer or wordprocessor is designed to print on a printing medium such as a thin plastic plate on the basis of image information including character information and the like.
Such printing apparatuses can be classified into the ink-jet type, wire-dot type, thermal type, laser beam type, and the like. Of the above printing apparatuses, an ink-jet type printing apparatus (ink-jet printing apparatus) is designed to print by discharging ink from a printing means such as a printhead onto a printing medium, and has the following advantages as compared with the other printing schemes. This printing apparatus allows an easy increase in resolution, can operate at high speed, and is very quiet. In addition, the printing apparatus is low in cost.
The need for color prints has increased, and many color ink-jet printing apparatuses have been developed. A general ink-jet printing apparatus uses a printhead formed by integrating pluralities of orifices and liquid channels as ink discharging portions as a printhead formed by integrating an array of a plurality of printing elements in order to attain an increase in printing speed. In addition, in order to realize color printing, such an ink-jet printing apparatus generally has a plurality of printheads.
FIG. 1 is a view showing the schematic arrangement of a general printer portion based on the scheme of printing by scanning a printhead on a printing sheet P. Referring to FIG. 1, reference numeral 101 denotes an ink cartridge. These ink cartridges are constituted by ink tanks respectively storing four color inks, i.e., black, cyan, magenta, and yellow inks, and identical printheads 102 provided for the respective inks.
FIG. 2 is a view showing the orifices formed in each printhead when viewed from the z direction. As shown in FIG. 2, a plurality of orifices 201 are arranged at predetermined intervals on the printhead 102.
Referring back to FIG. 1, reference numeral 103 denotes a convey roller for a printing medium, which rotates in the direction indicated by the arrow in FIG. 1 while holding a paper sheet P, together with an auxiliary roller 104 and sequentially feeds the paper sheet P in the y direction; 105, feed rollers for feeding a printing sheet and also holding the paper sheet P like the rollers 103 and 104; and 106, a carriage which supports the four ink cartridges 101 and moves/scans them in printing operation. These ink cartridges are set in the standby state at the home position (h) indicated by the dotted line in FIG. 1 while no printing is performed, or recovery operation is done for the printheads.
Before printing operation, when receiving a printing start instruction, the carriage 106 at the home position h in FIG. 1 discharges ink from a plurality of orifices 201 on the printhead 102 while moving in the x direction, thereby printing data. When data is completely printed up to an end portion of a printing sheet surface, the carriage 106 returns to the home position, and prints in the x direction again.
When an image or the like is to be printed, various factors need to be considered including color development characteristics, gray level characteristics, uniformity, and the like. With regard to uniformity, in particular, it is known that slight variations caused on a nozzle basis in a printhead manufacturing process will influence the amount of ink discharged from each nozzle and the discharge direction, resulting in a deterioration in image quality which appears as density irregularity of a printed image.
A specific example of this will be described with reference to FIGS. 3A to 3C and 4A to 4C. Referring to FIG. 3A, reference numeral 31 denotes a printhead constituted by eight nozzles 32; and 33, an ink droplet discharged from the nozzle 32. In general, it is ideal that ink is discharged with a uniform discharge amount in a uniform direction. If ink is discharged in this manner, dots with a uniform size land on a paper sheet as shown in FIG. 3B, and a uniform image without any density irregularity can be obtained as a whole (FIG. 3C).
In practice, however, each nozzle varies, as described above. If, therefore, printing is done in the above manner without any change, ink droplets discharged from the respective nozzles vary in size and direction as shown in FIG. 4A and land on a sheet surface in the manner shown in FIG. 4B. Referring to FIG. 4B, blank portions in each of which the area factor cannot be satisfied 100% periodically exist in the head main scanning direction, dots are excessively superimposed in some portions, and white streaks are produced as indicated at a central portion of this drawing.
A set of dots landed in this state exhibits the density distribution shown in FIG. 4C in the nozzle array direction. As a consequence, these phenomena are generally perceived as density irregularity by the human eye. In addition, if the convey amount of the printing medium varies, the resultant streaks may become noticeable.
As a countermeasure against density irregularity, the following method is disclosed in Japanese Patent Laid-Open No. 06-143618. This method will be briefly described with reference to FIGS. 4A to 4C and FIGS. 5A to 5C. According to this method, the printhead 31 is scanned three times in the main scanning direction (FIG. 5A) to complete the print area shown in FIG. 5B. A four-pixel area corresponding to ½ each print area is completed by two passes. In this case, the eight nozzles of the printhead are formed into two groups, i.e., four upper nozzles and four lower nozzles. The dot printed by one nozzle upon one main scanning operation corresponds to the data obtained by thinning out specified image data to about ½ in accordance with a predetermined image data arrangement (mask pattern). In the second main scanning operation, dots are formed in accordance with the remaining half image data to completely print a four-pixel area. The above printing method will be referred to as a multipass printing method hereinafter.
With the use of such a printing method, even if a printhead like the one shown in FIG. 4A is used, since the influences of the variations unique to the respective nozzles on a printed image are reduced to ½, an image similar to the one shown in FIG. 5B is printed. As a result, black and white streaks like those shown in FIG. 4B become less noticeable. As shown in FIG. 5C, the density irregularity is considerably reduced as compared with the case shown in FIG. 4C.
In such multipass printing, image data is divided into complementary data to be used in the first and second main scanning operations according to predetermined mask patterns. In most instances, patterns like staggered patterns in which pixels are vertically and horizontally staggered pixel by pixel as shown in FIGS. 6A to 6C are used as such mask patterns. In a unit print area (four-pixel area in this case), printing is completed by the first main scanning operation of printing a staggered pattern and the second main scanning operation of printing an inverse staggered pattern.
FIGS. 6A, 6B, and 6C show how printing in a predetermined area is done by using these staggered and inverse staggered mask patterns. First of all, in the first main scanning operation, printing is performed by using the four lower nozzles and the staggered mask pattern (FIG. 6A). In the second main scanning operation, the printing medium is conveyed by four pixels (½ the head length), and printing is performed by using the inverse staggered mask pattern (FIG. 6B). In the third main scanning operation, the printing medium is conveyed by four pixels (½ the head length), and printing is performed by using the staggered mask pattern again (FIG. 6C). In this manner, the printing medium is sequentially conveyed by four pixels at a time, and printing operations using the staggered and inverse staggered mask patterns are alternately performed to complete a four-pixel print area in each main scanning operation.
As described above, by completing an image in each print area using two different sets of nozzles, a high-quality image without density irregularity can be obtained.
There has recently been an increasing demand for an improvement in image quality in printing apparatuses. In order to meet this demand, attempts have been made to increase the resolution of printing apparatuses. If, however, the resolution of a printing apparatus is increased, the number of pixels increases, resulting in an increase in the amount of image data. This prolongs the data processing time in a host computer (host unit), the transfer time of data from the host computer to the printing apparatus, and the like.
The conventionally known matrix printing method is designed to solve such a problem. In this method, the image data processed in a host computer with a relatively low resolution by using many quantization levels (gray levels) is transferred to a printing apparatus, and printing is performed upon converting the received image data into print data corresponding to a predetermined dot matrix on the printing apparatus side. According to this method, even if the data amount is reduced, a gray level expression equivalent to the print result obtained by high-resolution processing can be realized.
In printing multilevel image data by multipass printing, an image is completed by scanning all areas (areas with different gray levels) the same number of times regardless of the quantization level (gray level) of the image data. However, the actual numbers of scans used to print at the respective gray levels differ from each other; the number of scans performed to actually print a low gray level portion, in particular, is small. That is, all the areas (areas with different gray levels) are scanned by the number of times (predetermined number of times) required to print a high gray level portion. However, the number of scans performed to actually print a low gray level portion is smaller than the predetermined number of times.
More specifically, when grayscale image data quantized with four quantization levels is to be printed by multipass printing with four passes, four scans are performed with respect to areas corresponding to the respective gray levels (level 1 to level 4). However, the numbers of scans performed to actually print the areas corresponding to the respective gray levels differ according to the levels. Data with level 1 is printed by one scan; data with level 2, by two scans; data with level 3, by three scans; and data with level 4, by four scans.
In this manner, proper printing with density irregularity and streaks being sufficiently reduced is done in a high gray level portion with a high quantization level, which rarely occurs in a natural image and the like, because printing is done by a relatively large number of scans. On the other hand, the same number of scans as in a high gray level portion with a high quantization level are also performed in a low gray level portion with a low quantization level which appears especially often in a natural image and the like. However, the number of scans used for actual printing is small, and hence unnecessary scans that actually print nothing are performed. More specifically, even if the same number (predetermined number) of scans as that for a high gray level portion are performed with respect to a low gray level portion, some of the predetermined number of scans are performed to actually print nothing. Since the number of scans that actually contribute to printing of a low gray level portion is small, the effect of multipass printing cannot be sufficiently obtained, and density irregularity and streaks tend to occur in a low gray level portion. This poses a problem (first problem).
Another problem is that in printing by assigning pixel patterns (dot matrixes) like those shown in FIG. 20 to the respective gray levels, when matrixes (pixel patterns) having different dot arrangements are assigned to the same low gray level (gray level 1), the intervals between the dots constituting a low gray level portion vary, resulting in graininess (noise).
This problem will be described by taking a specific example. Assume that dot matrixes (pixel patterns) each obtained by dividing a pixel into 2 (vertical)×1 (horizontal) portions are respectively assigned to gray level image data quantized with four values from level 0 to level 3 corresponding to the numbers of ink droplets, i.e., 0, 1, 2, and 4, to land within a pixel as shown in FIG. 20. In this case, data with quantization level 1 is assigned one of two kinds of dot matrixes, i.e., a dot matrix (the matrix indicated by “(B)” in FIG. 20) in which only one dot is placed on the left side and a dot matrix (the matrix indicated by “(C)” in FIG. 20) in which only one dot is placed on the right side. Data with quantization level 2 is assigned a dot matrix (the matrix indicated by “(D)” in FIG. 20) in which one dot is placed on each of the left and right sides. Data with quantization level 3 is assigned a dot matrix (the matrix indicated by “(E)” in FIG. 20) in which two dots are placed on each of the left and right sides.
FIG. 21A shows an image (low gray level portion) in which two kinds of dot matrixes corresponding to quantization level 1 are alternately arranged. As is obvious from FIG. 21A, the dot density is low (“coarse”) in a portion where a dot matrix (the matrix indicated by “(C)” in FIG. 20) in which only one dot exists on the right side is placed on the right of a dot matrix (the matrix indicated by “(B)” in FIG. 20) in which only one dot is placed on the left side. The dot density is high (“dense”) in a portion where a dot matrix (the matrix indicated by “(B)” in FIG. 20) in which only one dot is placed on the right side exists on the right of a dot matrix (the matrix indicated by “(C)” in FIG. 20) in which only one dot is placed on the left side. If coarse and dense portions are produced in this manner, the resultant image has graininess (noise). FIG. 21B shows an image (low gray level portion) constituted by two kinds of dot matrix patterns corresponding to quantization level 1 and one kind of dot matrix corresponding to quantization level 2. As is obvious from FIG. 21B, the dot density is low (“coarse”) in a portion where a dot matrix corresponding to quantization level 2 (the dot matrix indicated by “(D)” in FIG. 20) in which one dot is placed on each of the left and right sides exists on the right of a dot matrix corresponding to quantization level 1 (the matrix indicated by “(B)” in FIG. 20) in which one dot is placed on the left side. The dot density is high (“dense”) in a portion where a dot matrix corresponding to quantization level 2 (the dot matrix indicated by “(D)” in FIG. 20) in which one dot is placed on each of the left and right sides exists on the right of a dot matrix corresponding to quantization level 1 (the matrix indicated by “(C)” in FIG. 20) in which one dot is placed on the right side. In this case, as in the case shown in FIG. 21A, the production of coarse and dense portion leads to graininess (noise).
As described above, if a low gray level portion with a low quantization level which appears especially often in a natural image or the like is printed by using dot matrixes (pixel patterns) having different dot arrangements, the intervals between dots vary. This tends to cause graininess (noise). This poses a problem (second problem).